Liz (Tina Fey) preps for the end of 'TGS'. / Ali Goldstein, NBC

by Robert Bianco, USA TODAY

by Robert Bianco, USA TODAY

That may sound contradictory: "Funny" is, after all, one of the main qualities most viewers look for in a comedy. But while it may be necessary, it's seldom been sufficient, at least not to maintain an extended run.

That's why most popular sitcoms are built less on jokes than on characters: Their growth and change provides years' worth of plots, and on those weeks when the jokes falter, viewers can still enjoy spending time with the imaginary people they've come to love.

30 Rock, however, took a different approach. This once brilliantly amusing behind-the-TV-scenes satire, which ends its seven-year run Thursday night (8 ET/PT, NBC) with a one-hour special (** out of four), was always more interested in the industry it was skewering than in the characters working there. As you'd expect from a show created by and starring a former Saturday Night Live head writer, Tina Fey, 30 Rock thrived in those early, Emmy-winning seasons by making you think you were watching the biggest and best SNL running skit ever produced.

It also had a cast SNL would envy - at least when you looked beyond the show's fictional writers room. (Quick, name all those characters without checking IMDB.) There's no questioning the joy and skill provided over the years by Jack McBrayer, Jane Krakowski, Tracy Morgan and, most especially, Alec Baldwin. Each played a distinctly amusing character, each as broadly comic as a Marx Brothers creation - and each as unbelievable.

And that's where the problem came in. Change did come to 30 Rock's characters: As we greet Fey's Liz Lemon tonight, she's a stay-at-home wife and mother, having left her now-canceled late-night show, TGS With Tracy Jordan. But as the opening jokes make clear, nothing's really at stake in this change; you're not meant to emotionally invest in her marriage, and you don't.

For the first three years or so, that ironic approach worked. But when a show can't rely on characters' personal journeys to carry the load, it has to up the ante on the plots. And so with each season, Rock's plots got progressively and more gratingly silly, as if they were borrowed from the Telenovelas that once caused so many problems for Baldwin's Jack Donaghy. The quality fell, viewership dropped in half, yet the show rode on, dragging down a lineup that already had problems finding fans.

Anyone returning, hoping for the old Rock rather than the new, is bound to be disappointed in this strained outing, which says goodbye to TGS and by obvious extension 30 Rock itself. Crises glide by, old jokes return, and some of the show's worst tendencies go back on display, including its willingness to sacrifice the integrity of even its smartest characters for a stupid joke - or are we really to believe Jack wouldn't notice that the acronym for his latest plan makes a vulgar reference to the human posterior? There's even an attempt to make some kind sentimental, emotional contact, which feels like it was written for people who don't like, or don't have, emotions.

The producers sent a list of plot points they don't want spoiled, which we will respect, but really, would it matter if we didn't? If these characters all engaged in some massive murder/suicide pact (they don't) or were transported to Mars (they're not), would any viewer be particularly shocked? Anything can happen tonight, precisely because nothing matters, leaving you with an episode that will most likely convince those formerly loyal viewers that they were wise to drop out.

And that's what we're left with: a show that was great fun - just not nearly as long as it lasted.